1. Field of the Invention
The present invention relates to a drive circuit for a display device, and more particularly, to a display device drive circuit having a function for controlling the drive performance of an output amplifier.
2. Description of Related Art
A liquid crystal source driver for driving a liquid crystal display device includes a number of output amplifier circuits, and causes the output amplifier circuits to operate so as to drive a liquid crystal panel. In this case, the drive performance of the output amplifiers is adjusted depending on the type and usage of the liquid crystal panel. Meanwhile, in order to reduce heat generation of the display device and to reduce power consumption, a reduction in power consumption in an output amplifier unit is required.
Along with the recent increase in the screen size of liquid crystal display devices, the load capacitance of data lines of each liquid crystal display device is increasing. The liquid crystal source driver which outputs a display signal to a display output terminal of the liquid crystal display device is required to have a high drive performance when the potential of the display signal varies greatly. On the other hand, when the variation in potential of the display signal is small, the load of the data lines can be sufficiently driven with a low drive performance. Many liquid crystal source drivers are generally composed of output amplifier circuits having such features.
The magnitude of the variation in potential of the display signal varies depending on the display pattern of the liquid crystal display device. Thus, in the liquid crystal source driver, each of the output amplifiers has a function for setting a drive performance suitable for each display pattern.
FIG. 3 shows a circuit diagram of an output unit of a display device drive circuit (source driver) as a related art (see FIG. 15 of Japanese Unexamined Patent Application Publication No. 2007-156235). The circuit shown in FIG. 3 includes transistors 32a and 32b, which operate as constant current sources connected to an output transistor 31, and switch elements 33a and 33b. 
Assuming herein that grayscale signals of the display device are 8-bit digital signals of “D7, D6, D5, D4, D3, D2, D1, and D0”, D7 is defined as the most significant bit (MSB) and D0 is defined as the least significant bit (LSB). Additionally, the liquid crystal display device is assumed to be a normally black liquid crystal display device, since a large-scale liquid crystal display device is required to have a wide view angle and usually uses a normally black liquid crystal display device. In the normally black liquid crystal display device, the lowest transmittance (black display) is obtained when a voltage of 0 V is applied, and the highest transmittance (white display) is obtained when a voltage is applied. In this case, a grayscale signal of “00000000” is defined as indicating the black display, while a grayscale signal of “11111111” is defined as indicating the white display. The higher-order bits of the grayscale signal are used for the determination of a region of the white display or a region of the black display. In the circuit configuration of the output unit shown in FIG. 3, the most significant bit (D7) is used for the determination of a region of the white display or a region of the black display.
In the case where the most significant bit (D7) is used for the determination of the white display in which the variation in potential is large or the black display in which the variation in potential is small, at the time of the white display in which the most significant bit D7 is “1”, the switch element 33a is turned on and the switch element 33b is turned off, thereby causing both the transistors 32a and 32b to operate as constant current sources. Meanwhile, at the time of the black display in which the most significant bit D7 is “0”, the switch element 33a is turned off and the switch element 33b is turned on, thereby causing only the transistor 32a to operate as a constant current source. In this case, if the transistor 32a and the transistor 32b are equal in size, a constant current value obtained at the time of the black display is reduced to a half and the drive performance can be reduced. Moreover, this leads to a reduction in power consumption of the liquid crystal source driver without degrading the display quality.